﻿<p>The <em>IfcCurveBoundedSurface</em> is a parametric surface with boundaries defined by p-curves, that is, a curve
which lies on the basis of a surface and is defined in the parameter space of that surface. The p-curve is a special
type of a composite curve segment and shall only be used to bound a surface.</p>
<p>The outer boundary shall be either defined by:</p>
<ul>
<li>an <em>IfcOuterBoundaryCurve</em> a closed composite curve on surface for the definition of an outer boundary, in
this case the attribute <em>ImplicitOuter</em> has to be set to FALSE, or</li>
<li>an implicit boundary of the bounded surface, e.g. the u1, u2, v1, v2 of <em>IfcRectangularTrimmedSurface</em>, in
this case the attribute <em>ImplicitOuter</em> has to be set to TRUE.</li>
</ul>
<blockquote class="note">NOTE&nbsp; Some surfaces, like <em>IfcCylindricalSurface</em> does not have identifiable
implicit boundaries.</blockquote>
<blockquote class="extDef">NOTE&nbsp; Definition according to ISO/CD 10303-42:1992<br>
The curve bounded surface is a parametric surface with curved boundaries defined by one or more boundary curves. One of
the boundary curves may be the outer boundary; any number of inner boundaries is permissible. The region of the curve
bounded surface in the basis surface is defined to be the portion of the basis surface in the direction of
<b><em>N</em> x <em>T</em></b> from any point on the boundary, where <b>N</b> is the surface normal and <b>T</b> the
boundary curve tangent vector at this point. The region so defined shall be arcwise connected.</blockquote>
<blockquote class="note">NOTE&nbsp; Entity adapted from <strong>curve_bounded_surface</strong> defined in ISO
10303-42.</blockquote>
<blockquote class="history">HISTORY&nbsp; New entity in IFC4.</blockquote>
<p class="spec-head">Informal Propositions:</p>
<ol>
<li>Each curve in the set of <em>Boundaries</em> shall be closed.</li>
<li>No two curves in the set of <em>Boundaries</em> shall intersect.</li>
<li>At most one of the boundary curves may enclose any other boundary curve. If an <em>IfcOuterBoundaryCurve</em> is
designated, only that curve may enclose any other boundary curve.</li>
</ol>